(1) Field of the Invention
The present invention relates to a mass spectrometer, especially to a mass spectrometer suitable for mass spectrometry of ions which are formed by an atmospheric pressure ionization.
(2) Description of the Prior Art
A liquid chromatography directly coupled with mass spectrometry utilizing an electro-spray ionization (ESI), which is a type of atmospheric ionizations (LC/ESI-MS), has such a feature that fragment ions of a sample are less likely to be produced in comparison with a gas chromatography directly coupled with mass spectrometry utilizing conventional electron impact ionization (EI), because the sample is ionized moderately. In particular, observation of pseudo-molecular ions and multicharged ions in measurement of high molecules, such as peptide, can be facilitated.
In accordance with the LC/ESI-MS processes utilizing an ESI, a sample and a mobile phase, which are eluates from a liquid chromatograph, are supplied into an ESI probe through a capillary tube, and are neutralized at a top end of the probe with assistance from a nebulizer gas and a high electrostatic field. The nebulized sample molecules are ionized and accelerated under a high electric field so as to pass through a small hole in a first electrode, and are supplied into a medium pressure region which is formed between the first electrode and a second electrode.
At the beginning of the nebulization, the sample molecule is in the form of a small droplet in which a sample molecule is wrapped with the mobile phase (an eluent film for the sample), and is in a charged condition with a high voltage (about 5-8 kV) which is charged with the sample pipe.
A droplet of the nebulized sample is gradually reduced in size owing to evaporation of components in the mobile phase and ejects a sample molecular ion into a gas phase before reaching the first electrode.
That means, when the sample droplet becomes small, a Coulomb repulsive force between the sample ion and an electrostatic force in the mobile phase becomes larger than the surface tension of the mobile phase layer which exists on the surface of the sample particle, and the sample ions in the sample droplet are released from the mobile phase layer and change to ions of only sample molecules. The above phenomenon is called "ion evaporation".
The sample molecular ions are transferred to a stage for mass spectrometry through a small hole in the first electrode and a small hole in the second electrode, and are analyzed by the mass spectrometer. At the mass spectrometry stage, the analytical sensitivity is improved with the injected sample molecular ions having less dispersed distribution of masses.
Furthermore, the larger the ionization tendency of the sample, the higher will be the efficiency of the ion evaporation. Accordingly, the above mass spectrometer is suitable for analyzing a high polymer sample having strong polarities of NH, OH, and CO etc, such as a peptide, for example, with a high sensitivity. Consequently, the spectrometer has attracted a great deal of attention, especially in the field of medical analysis. On the contrary, an analysis of the above high polymer sample by a gas chromatograph direct connection type mass spectrometer (GC/MS) has been impossible because the sample is thermally destroyed easily.
However, actual sample molecules which are ionized by the ion evaporation inevitably still include a large number of mobile phase molecules , especially water molecules, and so sample molecular ions adsorbing water molecules pass through the small hole in the second electrode, although the water molecules are partially dissociated by collisions with neutral molecules when passing through the medium pressure region.
The sample molecular ions adsorbing water molecules collide with neutral particles in a free space at an entrance of an electric field for velocity dispersion in the mass spectrometry stage and a following electric field, and the water molecules are dissociated.
The number of the dissociated water molecules from a sample molecule have been calculated from the width of the kinetic energy of the sample molecular ions which pass through the electric field and are formed to be 30 to 60 molecules.
Similarly, the sample molecular ions dissociate water molecules by collisions with neutral particles in the space from an outlet of the electric field to an outlet of a magnetic field for mass dispersion.
Although a large number of water molecules are dissociated in a manner described above before reaching the entrance of the magnetic field, if the residual amount of water molecules is large, a dispersion of distribution in mass of the sample molecular ions becomes large, and consequently, the analytical sensitivity decreases because not all of the sample molecular ions are able to reach the detector.
A method which proposes to solve the above described problem is disclosed in U.S. Pat. No. 4,977,320 (Chowdhury, Katta, and Chait). In accordance with Chowdhury et al, a capillary tube which is wound with a heater is installed in front of the first electrode, and a nebulized sample which is injected from an ESI probe is introduced into the capillary tube.
The capillary tube is located between an atmospheric pressure region and a reduced pressure region, and accordingly, restricts gas flow into the reduced pressure region by its flow resistance, concurrently extends the residence time for the nebulized sample which is injected from the ESI probe to reach the small hole in the first electrode, and increases the efficiency of ion evaporation. As a result, an effect for homogenizing the mass of the sample molecular ions is assumed to be increased.
However, the temperature of the nebulized sample decreases while passing through the capillary tube as a result of adiabatic expansion, and consequently, the efficiency of the ion evaporation decreases. But, this undesirable affect on the efficiency can be overcome by elevating the temperature of the nebulized sample with the heater which is wound around the capillary tube.
For instance, in a case of the ESI mass spectrometer disclosed in U.S. Pat. No. 4,977,320, a capillary tube having 0.5 mm in inside diameter and 10-20 cm in length is used, but fabrication of such a capillary tube is very difficult in practice.
Furthermore, washing of the capillary tube necessary for maintenance is difficult, fixing of the capillary tube and an axial adjustment of the tubes during that replacement are troublesome, and the winding of the heater on the capillary tube is difficult. Additionally, a biomolecule sample, such as a peptide, decomposes easily by over-heating, and consequently, it is necessary to keep the heating temperature of the capillary tube below approximately 80.degree. C. when the sample passes through a long capillary tube for a relatively long time.